Electrostatic methods and apparatus for mounting and demounting particles from a surface having an array of tacky and non-tacky areas

ABSTRACT

Methods and associated apparatus are disclosed for use in mounting particles on and de-mounting particles from a substrate having an array of tacky and non-tacky areas. The particles can be either electrically conducting or electrically non-conducting. Selection of electrically conducting particles is preferred. The substrate having an array of tacky and non-tacky areas can either be electrically non-conducting (e.g., a dielectric substrate) or electrically-conducting. The methods involve use of first and second electrode plates with the substrate therebetween, the plates having applied thereto a direct current potential, which potential in preferred embodiments is reversed in polarity for a number N of cycles. Methods and articles are disclosed using an electrically conductive surface adjacent the tacky and non-tacky areas to minimize static buildup on the particles and tacky and non-tacky areas.

This is a divisional of application Ser. No. 10/322,283, filed on Dec.17, 2002 now U.S. Pat. No. 6,871,777, which is a divisional ofapplication Ser. No. 09/876,237, filed on Jun. 7, 2001, which issued asU.S. Pat. No. 6,540,127 on Apr. 1, 2003, and which claims priority toprovisional application Ser. No. 60/213,128, filed on Jun. 22, 2000, nowabandoned.

FIELD OF THE INVENTION

This invention relates to an improved method for transporting particlesfrom one surface to another using electrode plates having a directcurrent potential difference between them. The invention also relates tomethods and articles for minimizing static buildup on particles andsurfaces.

TECHNICAL BACKGROUND OF THE INVENTION

The placement of particles, such as electrically conductive solder, oncontact pads is critical to the adoption of array style semiconductorpackages such as ball grid arrays (BGA). Such placement is also criticalin the attachment of integrated circuits (IC) to packages or printedcircuit boards through flip chip processes. Recent attempts have beenmade to improve, for example, solder ball interconnects, such that morereliable and/or less costly solder connections are made in electronicapplications. Despite these efforts, there are still problems associatedwith the handling and transfer of particles, primarily conductiveparticles such as solder balls to form solder bumps, on the contact padsof electronic devices. There is a need for further improvements,particularly with regard to the efficiency, precision, and robustness ofthe process.

SUMMARY OF THE INVENTION

The present invention is a method for transferring particles from anelectrode plate to tacky areas present on a substrate comprising:

-   -   a) placing a substrate having both tacky and non-tacky areas        between first and second electrode plates, the substrate and        electrode plates arranged substantially horizontally and stacked        substantially vertically, wherein the first electrode plate        -   (i) lies below the substrate,        -   (ii) has a surface which faces tacky and non-tacky areas on            the substrate, and        -   (iii) is spaced from the substrate and the second electrode            plate;    -   b) applying particles over the surface of the first electrode        plate; and    -   c) applying a direct current potential between the first and        second electrode plates for a time T₁, establishing a polarity        on the first electrode and thereby causing the particles to be        charged and be propelled toward the second electrode plate,        resulting in at least a portion of the charged particles        becoming adhered to tacky areas on the substrate.

Another embodiment of the invention having additional step(s) includesas step d) changing the direct current potential on the first electrodeplate for a time T2 after step c) to cause at least some of theparticles to leave non-tacky areas of the substrate, be propelledagainst the first electrode, again be charged and be propelled towardthe second electrode. Still other embodiments include repeating steps c)and d) for a number, N, of cycles as step e), eliminating the directcurrent potential between the electrode plates and removing particlesfrom the non-tacky areas as steps f) and g), and placing anon-conductive shield between the substrate and the first electrodeplate as step h).

In another embodiment, the invention is a method for mounting particleson a substrate having both tacky and non-tacky areas thereon, wherein adirect current potential between first and second electrode plates isused in the method and in which the particles are first applied to asubstrate having tacky and non-tacky areas, which substrate is placedover the first electrode.

In further embodiments, the tacky areas are heated to improve adhesionand centering of the particles in the tacky areas. The invention alsocomprises apparatuses for practicing the above methods. The inventionalso comprises an article having a substrate or surface with tacky andnon-tacky areas that has an electrically conductive surface adjacent tothe tacky and non-tacky areas to dissipate electrostatic charges and amethod for changing the tacky and non-tacky areas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified section view of one embodiment of a surfacehaving an array of tacky and non-tacky areas thereon, which surface issuitable for use with the inventive methods, wherein the tacky andnon-tacky areas are disposed coplanar with one another.

FIG. 2 is a simplified section view of another embodiment of a surfacehaving an array of tacky and non-tacky areas thereon, which surface issuitable for use with the inventive methods, wherein the tacky areas aredisposed below the plane of the non-tacky areas.

FIG. 3 is a simplified section view of still another embodiment of asurface having an array of tacky and non-tacky areas thereon, whichsurface is suitable for use with the inventive methods, wherein thetacky areas are disposed above the plane of the non-tacky areas.

FIG. 4 is a simplified section view of the array of FIG. 1, shown incombination with a particle adhered to each tacky area.

FIG. 5 is a simplified section view of the array of FIG. 2, shown incombination with a particle adhered to each tacky area.

FIG. 6 is a simplified section view of the array of FIG. 3, shown incombination with a particle adhered to each tacky area.

FIG. 7A is section view of a spherical particle initially adhering to atacky area on a substrate.

FIG. 7B is a plan view of FIG. 7A looking through a translucentsubstrate and tacky area.

FIG. 7C is the section view of FIG. 7A after a predetermined dwell timewhen the condition is that the spherical particle contacts the substratebefore contacting the complete circumference of the tacky dot.

FIG. 7D is a plan view of FIG. 7C looking through a translucentsubstrate and tacky area.

FIG. 7E is an alternative section view of FIG. 7A after a predetermineddwell time when the condition is that the spherical particle contactsthe complete circumference of the tacky dot before the particle contactsthe substrate.

FIG. 7F is plan view of FIG. 7E looking through a translucent substrateand tacky area.

FIG. 8 illustrates the geometrical relationships involved forself-centering of a sphere of diameter 2r in a tacky area of thickness zwith contact diameter x and with the sphere penetrating all of the tackyarea and resting on the substrate at the bottom of the tacky area.

FIG. 9 is a schematic of an apparatus for mounting particles on asubstrate having an array of tacky and non-tacky areas thereon, whereinthe substrate is a discrete portion of web.

FIG. 10 is a schematic of an apparatus for mounting particles on asubstrate having an array of tacky and non-tacky areas thereon, whereinthe surface is a continuous elongated web.

FIG. 11 is a schematic of the equipment used in Example 6 demonstratingthe effect of a conductive substrate.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to an improved method and apparatus based uponelectrostatics for precisely and efficiently adhering particles to tackyareas on a surface of a substrate having an array of tacky and non-tackyareas and removing particles from the non-tacky areas without removingparticles from the tacky areas. This net adhering of particles to thetacky areas on the surface is termed net population of the substrate.For most applications of this invention, it is desired that there be oneand only one particle attached to each tacky area of the substrate.

In one embodiment of this invention, the substrate and particles areplaced between two electrodes and an electric field is applied to propelthe particles onto the substrate to populate the tacky areas. In anotherembodiment, the electric field is applied to remove particles from thenon-tacky areas of the substrate after populating the tacky areas. Themethod and apparatus are useful with both electrically conductive andnon-conductive particles and substrates.

Cairncross et al., U.S. Pat. No. 5,356,751, which is incorporated byreference discloses a process and product for mounting free-flowingparticles, which employs a support having a support surface with anarray of tacky areas which have a size and bonding strength suitable foradhesion of either one or two of said particles. In the process, theparticles flow across the support surface to allow particles to contactthe tacky areas and adhere thereto.

Substrates having Arrays of Tacky and Non-Tacky Areas and AssociatedMethods

The array and method described herein are particularly suited for usewith free-flowing particles. By free-flowing is meant that there is nosubstantial binding force to be overcome when separating a mass ofparticles into separate discrete particles and that the particles do notstick to one another or clump together under normal conditions of use. Adiscussion of particle to particle binding forces is presented in U.S.Pat. No. 5,356,751.

For most electronic applications, the preferred particles for use inconnection with this invention are electrically conductive materials,such as Cu, In, Pb, Sn, Au, and alloys thereof. Most preferred aresolder balls. It will be apparent to those skilled in the art, however,that the type of particle used in connection with the present inventionis dictated by the particular application and is not an inherentlimitation of the invention. For example, a particular application mayrequire that an electrically insulating material be applied to a solderbump on a contact pad; e.g., to space one contact pad from another in astack of circuit boards. The present invention may be used to advantagein such circumstances. Generally speaking, spherical particles will bepreferred in the practice of this invention because of their ease inhandling and particle symmetry, however, the size and shape of theparticles are not critical to the invention. For example, slightlyoff-round particles, such as seeds, work well with this invention.

For other applications outside the electronic field, the particles canhave properties without any particular size or shape limitations exceptfor the limitation that the particles must have sufficient compatibilitywith the tacky areas such that the adhesive force bonding each particleto a given tacky area is at least the minimal value specified herein(i.e., at least 2 grams/mm2). The particles, such as beads, can eitherbe electrically-conductive or electrically-non-conductive such as glass;organic, inorganic, organometallic, or mixtures thereof; polymeric ornon-polymeric; and living or non-living. Examples of suitable particlesfor this invention include, but are not limited to, mineral grains,chemical products, salt and sugar granules, polymer particles,mechanically ground solids, pollen, spores, and seeds. Some specificchemical product particles are solder, alumina, and silica; a specificmineral grain is glass. Some specific polymer particles arepoly(styrene), poly(methylmethacrylate) and poly(ethylene). Organic,inorganic, or organometallic chemical compounds that arepharmaceuticals, herbicides, pesticides, or have other biologicalactivity are suitable particles for this invention; these compounds canbe present at levels lower than or equal to 100% of the particlecomposition. If lower, other components can be present in the particleswithout limit. The particles can comprise any gas(es) and/or liquid(s)compounded with (e.g. absorbed on) any solid(s). For example, particlescomprising dimethylsulfoxide (a liquid) absorbed onto alumina (a solid)are suitable in this invention.

As used herein, the term tacky areas means areas, volumes, or regionshaving adhesive properties to enable a bond to form immediately uponcontact with free-flowing particles under low pressure (e.g., the weightof the particles or the wetting action between the adhesive and theparticles). The areas have a thickness that plays a role in centeringthe particles and maximizing the surface area for adhesive contact. Inaccordance with this invention, the tacky areas have a size and bondingstrength suitable for adhesion of one particle per tacky area.Typically, the tacky areas are small shapes (i.e., dots) from about 0.25um to 1000 um and for many embodiments they are from about 10 um to 500um. The tacky area shapes may be circular, square, rectangular, oval, oranother shape suitable for retention of the particle. Generally,circular shaped tacky areas are preferred.

The spacing of the tacky areas is such that the position of one particleon one tacky area relative to the position of other particles onadjacent tacky areas matches the distance between and relative positionof the contact pads of the electronic (or other) device to which theparticles will be transferred and become attached. The actual dimensionsused for the tacky area spacing and the contact pad spacing must takeinto account differences in thermal expansion that may occur between thematerial of the substrate and the material of the electronic device. Atthe temperature used during transferring, the spacings should match. Thelocation of the tacky areas at least must allow the particles to touchsome part of the contact pad to which it will become attached. In theembodiment where the particle melts (e.g., solder particles in contactwith solder flux and a metallized contact pad), direct contact isrequired between the molten particle and the pad so that the moltenparticle can wet the pad and flow across the metal surface to cover themetallized pad. The initial contact of the particle with the metallizedcontact pad may be off-center because the wetting action of the moltenparticle will center the particle over the pad during attachment. Forthese noncritical embodiments, the original pattern of tacky areas issuch that the location of each tacky area must align and overlapsomewhere within the area of the corresponding contact pad area to whichit will attach, and the size of the tacky area must be smaller than theparticle so that only one particle is attached to each tacky area.Typically, for a tacky area having a particular size and bondingstrength, there is an upper limit to the size and weight of particle,above which there is no substantial particle adherence, and there is alower limit to the particle size which will adhere singly to each tackyarea. For tacky areas with a tackiness of 2 to 6 grams/mm2 and particlesof 0.127 to 0.762 mm (0.005 to 0.030 inch) diameters, the tacky area maybe as small as 15% of the particle diameter to as large as 100% of theparticle diameter and still get single particle attachment per tackyarea. A tacky area of 30 to 60% of the particle diameter is preferred.

In cases where the contact pads are close together relative to the sizeof the particle, care must be taken so that the particles on adjacenttacky areas do not touch before and during attachment to the contactpads so as to avoid bridging adjacent contact pads. As the space betweencontact pads become smaller relative to the width of the pad and hence,to the width of the particle to be attached to the contact pad, itbecomes critical to align the tacky areas and particles closer to thecenter of the matching contact pads to which the particle will becomeattached. This is accomplished by centering the tacky area positions inthe imaging step to match the center of the contact pads and using acombination of smaller tacky areas and an optimum combination of tackyarea thickness and diameter for the particular surface curvature of theparticle to achieve self-centering of the particle in the tacky area(see later discussion of self-centering).

Single particle attachment to each tacky area is assured when the sizeof the particle is large enough to cover the tacky area upon attachment,thus preventing further particles from ever touching the tacky area ofan occupied tacky area. For the preferred embodiments of sphericalparticles and circular tacky dot areas, this is achieved once thediameter of the tacky dot area is less than the diameter of the smallestparticle. A narrow size range for the particles is also desired tocontrol the volume after the particle is attached to the contact pad. Auniform particle diameter is also desired for good contact betweenparticles attached to tacky areas on a transfer substrate and thecontact pads of the electronic device to which the particle is to betransferred. A size range of +/−10% for the particle diameter ispreferred.

The array of tacky and non-tacky areas preferably has clearly definedtacky areas and has no foreign material adhered thereto. Preferably, thenon-tacky areas are flat and smooth and are either disposed coplanarwith the tacky areas or the tacky areas are disposed below the plane ofthe non-tacky areas. Most preferably, the non-tacky areas are flat andsmooth and are disposed co-planar with the tacky areas. Although lesspreferred, the tacky areas may be disposed above the plane of thenon-tacky areas. In each of these cases, there can be material at theinterface of a given tacky area with the non-tacky area that is slightlyout of plane right at the interface (either above or below the plane ofthe interface even starting with a coplanar substrate prior to imagingto form the array of tacky and non-tacky areas). While not being boundby any theory, it is believed in the case of a photopolymer layer thatthis effect results from the diffusion of unpolymerized components fromthe tacky areas into the non-tacky areas thickening the border aroundthe tacky areas. Also lightly crosslinked tacky areas are less dense andslightly thicker than more highly crosslinked non-tacky areas.

In a particularly preferred embodiment, the array of tacky and non-tackyareas comprises a photosensitive element that has been imagewise exposedto create the array. A variety of positive and negative photosensitivecompositions are known to produce tacky images and may be used in thepractice of this invention. Phototackifiable compositions become tackywhere struck by light and are exemplified by compositions described inU.S. Pat. No. 5,093,221, U.S. Pat. No. 5,071,731, U.S. Pat. No.4,294,909, U.S. Pat. No. 4,356,252 and German Patent No. 3,514,768.Photohardenable compositions are those which become hardened in lightstruck areas. A number of photohardenable compositions include Cromalin®Positive Proofing Film SN 556548, Cromalin® 4BX, Surphex™ (embossablephotopolymer film, Cromatone® Negative Overlay Film SN 031372, andCromaling Negative Film C/N all available from E. I. du Pont de Nemoursand Company, Wilmington, Del. Cromalin® Positive Film SN 556548,Cromalin® 4BX and Surphex™ are preferred. These and other photosensitiveproducts are disclosed in U.S. Pat. No. 3,649,268, U.S. Pat. No.4,174,216, U.S. Pat. No. 4,282,308, U.S. Pat. No. 4,948,704 and U.S.Pat. No. 5,001,037.

Photohardenable compositions are generally a combination of polymericbinder and photopolymerizable monomers. Suitable binders includeco(methyl methacrylate/methacrylic acid) and monoethyl ester ofpoly(methyl vinyl ether/maleic anhydride), each of which may becopolymerized in various proportions. Suitable photopolymerizablemonomers include ethylenically unsaturated monomers which have beenfound useful are those disclosed in U.S. Pat. No. 2,760,863; U.S. Pat.No. 3,380,831 and U.S. Pat. No. 3,573,918. Examples aredipentaerythritol acrylate (50% tetra and 50% penta), pentaerythritoltriacrylate and tetraacrylate, polypropylene glycol (50) ether ofpentaerythritol tetraacrylate, polyethylene glycol (200) dimethacrylate,dipentaerythritol triacrylate b-hydroxyethyl ether, polypropylene glycol(550) ether of pentaerythritol tetramethacrylate, pentaerythritoltetramethacrylate, polypropylene glycol (425) dimethacrylate,trimethylolpropane trimethacrylate, and polypropylene glycol (340) etherof trimethylol propane triacrylate. (Note: Numbers within parentheses inthis paragraph, e.g., 550, 425, 340, and 50, are number averagemolecular weights.) Also useful are epoxy monomers containing ethyleneunsaturation, e.g., monomers of the type disclosed in U.S. Pat. No.3,661,576 and British Patent No. 1,006,587. The binder may be variedwidely in its ratio with the monomer but in general it should be in therange of 3:1 to 1:3. The monomer should be compatible with, and may be asolvent for, and/or have a plasticizing action on the binder. The choiceand proportions of monomer and binder are made in accordance with therequirements of selective photoadherence.

When the pattern of tacky areas is not used immediately, or is stored orshipped, it is useful to keep the tacky areas clean by protecting themwith a cover sheet such as a polyester film, polypropylene film, orsilicone release polyester film. Generally a thin 0.0127 mm (0.0005inch) Mylar® polyester film (E. I. du Pont de Nemours and Company,Wilmington, Del.) is sufficient.

When using photosensitive compositions to create the array of tacky andnon-tacky areas, the photosensitive composition is first applied to asuitable substrate and is then imagewise exposed to create the desiredarray of tacky and non-tacky areas. As discussed more fully below, thechoice of substrate will largely depend upon the method selected tomount the array of particles to the contact pads. Generally speaking,however, the substrate should be stable under the conditions of intendeduse, smooth, and show good adherence to the photosensitive composition.As will be recognized by those skilled in the art, one or moreintermediate layers may be applied to the substrate to improve adhesionof the photosensitive layer. In one embodiment of this invention, thephotosensitive composition is applied to a metallic layer (or to a layerof another material that is electrically-conductive) to afford apreferred article of this invention—see detailed discussion infra.

There should be facile control of the tacky areas with respect to sizeand placement. For the aforementioned photosensitive products, the arraypattern is first composed by manual or computer assisted design, and isusually transferred to a photographic film that is used as a photo toolin contact with the photosensitive product and with strong ultravioletlight to pattern the tacky array in the photosensitive product. For theCromalin® products, the photosensitive material would first be laminatedto or coated onto the substrate and then exposed through the phototoolto create the pattern. The pattern could be made to coincide with theinterconnect positions of a circuit board. For Cromatone®, a clearplastic film substrate is provided with the product so that it may beexposed directly through the phototool. Other patterning methods includeprojection exposure and direct writing as in digital imaging using alaser output device.

With reference now being made to FIG. 1, an article or web 8 having anarray of tacky and non-tacky areas suitable for use in accordance withthe process of the invention is illustrated therein. In the embodimentshown, the article comprises a photosensitive layer 10 applied to asubstrate 12. The photosensitive layer 10 has been imagewise exposed toproduce alternating areas 14 which are non-tacky and areas 16 which aretacky. If the photosensitive layer 10 is a phototackifiable composition,the areas 16 would correspond to the exposed areas whereas if thephotosensitive layer 10 is a photohardenable composition, areas 16 wouldcorrespond to the unexposed areas.

The substrate as shown in FIG. 1 may itself be electricallynon-conductive or electrically conductive, or a conductive layer can bepresent as a separate layer 12 a on a substrate base 12 b, in which casenormally the conductive layer 12 a would be between the photosensitivelayer 10 and the substrate base 12 b. The conductive material may beuseful in controlling electrostatic charge on the web. The conductinglayer is comprised of an electrically-conducting material. Suitablematerials that are electrically-conducting include, but are not limitedto, metal(s), metal oxide(s), and electrically-conducting polymers.Suitable metals or metal oxides include, but are not limited to,aluminum, copper, and indium tin oxide. Electrically-conducting polymersmay also be used which may include polymers containing very fineelectrically conductive particles, such as carbon particles.

The conducting layer can either be present as a separate layer 12 a thatis adjacent to and in contact with a substrate 12 b, or, alternatively,the conducting layer can be present without a separate substrate, inwhich case it serves as its own substrate 12. In the case of the former,the conducting layer 12 a can be applied to the substrate 12 b by anymethods known to the art, which include, but are not limited to,coating, vacuum deposition, and electroless plating. In the case of thelatter, illustratively, one example where the surface and the conductinglayer are present in one layer is a layer of copper or aluminum (e.g.,copper foil or aluminum foil) of sufficient thickness such that thelayer simultaneously serves as the substrate 12 as well as being theconducting layer. In a further alternative, the conductive layer can bepresent on the substrate as part of the tacky and non-tacky area, thatis, the tacky and non-tacky areas themselves may be electricallyconductive.

An alternative to the tacky and non-tacky areas being a phototackifiablecomposition, is for the article to be formed by attaching a thin sheetmaterial having an array of holes to an adhesive coated substrate.Examples of such sheet material include screen mesh or stencils whereinholes have been formed by, for example, laser ablation, punching,drilling, etching, or electroforming. The article may also be formed byproviding photoresist hole patterns on an adhesive coated substrate. Anexample of such an alternate article or web 108 is illustrated in FIG.2, wherein an adhesive layer 110 is applied to a substrate 112. Thesubstrate can itself be non-conductive or conductive, or a conductivelayer can be present as a separate layer on a non-conductive substrate,in which case normally the conductive layer would be between thephotosensitive layer and the substrate. The tacky and non-tacky areasthemselves may be conductive. A thin sheet material 114 having holes 116therein is then applied over the adhesive layer 110. The adhesive layer110 is an outer surface in the areas of the holes 116 in the sheetmaterial 114, thereby affording the tacky areas. It will be apparent tothose skilled in the art that a similar type of structure, that is, anon-tacky surface having recessed tacky areas, will also result from theuse of certain photosensitive materials (e.g., negative Cromalin® orCromotone®) which produce a peel-apart image. Generally, the further thetacky area is recessed in relation to the non-tacky area, the morelikely size exclusion will occur, where no particles larger than thewidth at the tacky area recess, will attach. This effect becomesparticularly pronounced as the tacky area recess approaches the size ofthe tacky area, that is, the depth of the tacky area is approximatelyequal to its width.

With reference now being made to FIG. 3, still another embodiment of anarticle or web 208 having an array of tacky and non-tacky areas suitablefor use in accordance with the process of the invention is illustratedtherein. In the embodiment shown, the article 208 comprises an array oftacky areas 216 on a non-tacky substrate 212.

It is noted that in the embodiment shown in FIG. 1, the tacky areas 16are disposed co-planar with the non-tacky areas 14 whereas in theembodiment of FIG. 2, the tacky areas, corresponding to holes 16, aredisposed below the plane of the sheet material 114, which defines thenon-tacky areas. It is further noted that in the embodiment shown inFIG. 3, the tacky areas 216 are disposed above the plane of thenon-tacky conductive layer 212.

This invention relates to improved articles and methods for efficientlyand precisely adhering particles to the tacky areas (as described above)on a surface containing an array of tacky and non-tacky areas andremoving particles from the non-tacky areas without removing theparticles from the tacky areas. The improved articles and methods arediscussed in depth in the section following this one.

Following the population process, in many applications for thisinvention, the array of mounted particles described above is transferredto contact pads of an electronic device. The contact pads are usuallymade of a conductive metal such as copper, aluminum, gold, or a lead/tinsolder. In a preferred method of transfer of the mounted particles, anarray having a single conductive (e.g., solder) particle adhered to thetacky areas thereof is placed in contact with the contact pads of anelectronic device such that the particles are placed in registeredcontact with each of the contact pads and the particles are thenreleased from the tacky areas of the array and are adhered to thecontact pads. The array of tacky and non-tacky areas may themselves beelectrically conductive or they may be on a substrate surface and have aconductive surface adjacent to these areas. This method will be referredto as the transfer method. In an alternate method of transfer, the arrayof tacky and non-tacky areas is formed directly on the contact pads(such as by coating, laminating etc.) prior to the particles beingadhered thereto. Once again, the array of tacky and non-tacky areas maythemselves be electrically conductive or they may be on a surface layerand have a conductive surface adjacent the tacky and non-tacky areas;the contact pads where the tacky areas are located are already, bydefinition, electrically conductive and may serve as the conductivelayer adjacent the tacky areas. This method will be referred to as thedirect method.

In either the transfer method or the direct method, it is necessary todisassociate the particles from the tacky areas of the array. There aremany alternate methods to accomplish this step, some of which are moreapplicable to either the direct method or the transfer method than tothe other. For example, disassociation of the particles can beaccomplished by mechanical forces, that is, an adhesive compound (e.g.,a viscous flux, acting as an adhesive or having an adhesive component)can be applied to the contact pad. Upon contact of the solder ball tothe adhesive compound, a bond forms which is stronger than the bondbetween the solder ball and the tacky area of the array. Thus, uponremoval of the array from the contact pads, the particles are releasedfrom the tacky areas and remain adhered to the contact pads. Mechanicaldisassociation of the particles is particularly applicable to thetransfer method.

Thermal disassociation is yet another method of disassociating theparticles from the array. By thermal disassociation is meant theapplication of heat sufficient to cause the particles to melt, wet thesurface of the contact pads and flow to cover the pads. Preferably, asthe particles melt, the substrate is brought closer to the contact padsto make sure that all particles contact their respective contact pads.Spacers may be used to keep the surface uniformly off contact from thecontact pads themselves so as not to squeeze solder beyond the contactpads.

The heat necessary to melt the particles may be provided by use of anoven, laser, microwave, infrared radiation or other convenient source.Temperatures in the range of 150° C. to 400° C. are normally sufficientto cause the reflow of the particles, particularly solder balls. It willbe apparent to the skilled artisan that, in the event the substrate willbe heated together with the particles, the substrate should be capableof withstanding such temperatures; that is, it should be thermallystable. Non-conductive substrates, such as Kapton® (a polyimide filmavailable from E. I. du Pont de Nemours and Company, Wilmington, Del.),quartz, glass and the like, may be used to advantage. Likewise, withregard to the material used to form the tacky and non-tacky array, suchmaterial should not melt during the heat step, but rather should bethermally stable or, alternatively, should completely volatilize at suchtemperatures. Negative Cromalin® in particular has a tendency to meltduring an oven heating disassociation step and thus is largelyunsuitable for use with oven heating. In the event that the heat sourceused will not heat the substrate or tacky and non-tacky areas (e.g., alaser), thermal stability is not of great concern.

Another method that may be used to disassociate the particles isphotodisassociation. In this method, the tacky areas are exposed toactinic radiation whereby they lose their adhesive properties todisassociate the particle.

To improve the wetting and adhesion of the particle, particularly solderballs, to the contact pads, a suitable flux may be used. A solder fluxcombination (e.g., rosin types, no-clean types, organic acid orsynthetic activated) can be coated on the pads areas and/or on thesolder balls to help clean oxide layers from the pad and solder,improving wetting of the metallized pad by molten solder therebyeffecting disassociation of the solder ball from the tacky area andadhesion thereof to the contact pad.

In the direct method, it is critical that the molten particledisassociate or displace the tacky area on the contact pad andcompletely wet the contact pad with the molten particle (e.g., solder).This could be accomplished by decomposing the tacky areas to volatilecompounds when the melting temperature of the particles is reached or byusing thermally stable tacky area materials that would be displaced bythe molten particle.

Once the particles have been released from the tacky areas and melted,they are allowed to cool and resolidify on the contact pads, e.g., toform a solder bump.

Population

This invention encompasses electrostatic methods for mounting anddemounting particles from a surface having an array of tacky andnon-tacky areas. In populating the array of tacky and non-tacky areas,it is desired to effect placement of a controlled number of particles oneach tacky area while ensuring that there are no excess particlesremaining in any area that is non-tacky, at the end of the populationprocess. Most often for electronic applications in particular, it isdesirable to place precisely one particle on each tacky area.

In general, the population step may be accomplished in a number of ways.Generally the article with the pattern of tacky areas is placed in acontainer with an excess of particles and the container gently moved soas to allow the particles to move across the array until all tacky areasbecome occupied. Alternatively, excess particles are sprinkled onto thetacky areas until all tacky areas are covered with particles. Excessparticles are removed from the fully occupied pattern of tacky areas bygravity, gentle tapping, gentle blowing, vacuum and other methods. Theforce used in the clean up of excess particles depends on the adhesivestrength of the bond between the tacky areas and the particles. Thisstep, the application of free flowing particles to patterns of tackyareas, is accomplished best when electrostatic charging is avoided byusing electrically conducting, grounded containers, humidifiedatmosphere and with the use of ion generators, as in the use of ionizedair. This step is further aided by a clean atmosphere to prevent theattachment of foreign matter to the tacky areas.

FIGS. 4, 5, and 6 illustrate the different cases of the arrays shown inFIGS. 1, 2, and 3 respectively, with spherical particles 20 attached tothe tacky areas 16, 116, and 216 of the array to form populated articlesor webs 8 p, 108 p, and 208 p, respectively.

The figures above showing particles attached to tacky areas of an arrayof tacky and non-tacky areas are schematic. It should be understood thatthese figures depict representation(s) not-to-scale. In actual practiceof this invention, typically particles initially attach to tacky areasnear the perimeter of the tacky area with relatively light wetting ofthe particle by the tacky area. Later, at equilibrium wetting, typicallythere is full or nearly full embedding of particles in the tacky areaswith centering of the particles.

The process of attaching particles to patterns of tacky areas is aidedby the tacky areas having sufficient tackiness to grab and hold theparticles immediately upon contact. It is further desired for theattachment of the particle to the tacky area to be strong enough towithstand the various forces (e.g., vibrating, tapping, shaking,jiggling, moving, bumping contact, vacuum or blowing forces,electrostatic propulsion, etc.) that occur while populating the arraywith particles and during the removal of excess particles from the fullypopulated array. In a preferred method vibrating is employed at notgreater than 1000 cycles per minute to distribute particles over thetacky and non-tacky areas and dislodge particles from the non-tackyareas. In addition, it is advantageous to have sufficient adhesivestrength between the particles and the tacky areas to hold the particlesin place during handling and possible shipment. Furthermore, tacky areaswith a tackiness of at least 0.5 grams/mm2 can be populated byparticles, but it is preferred that the tacky areas have a tackiness ofat least 2 g/mm2 and it is most preferred that the tacky areas have atackiness of at least 5 g/mm2, especially when patterns of tacky areaspopulated with particles are to be shipped without loss of theparticles.

The goal is perfect population with one particle per tacky area and noextras; with total errors (TE) per populated article of zero. Expressedas an equation:TE=V+TW+EX

-   where TE=total errors per article-   V=total number of vacant tacky areas per article-   TW=total number of extra particles associated with a tacky area or    “twins” per article-   EX=total number of extra particles left on non-tacky areas per    article

Then the error rate ER for the populated surface becomesER=1,000,000(TE)/TAwhere ER=error rate in parts per million (ppm) tacky areas

-   TA=total number of tacky areas per article-   TE=total errors per article

For almost all tacky areas small particles will attach to the edge ofthe tacky areas as soon as the particles flow across the tacky areasprovided that the kinetic energy of the particle is less than theinitial bonding strength of the particle to the tacky area. Once thetacky areas are buried with excess particles at rest the number ofvacancies V is very low. Remaining vacancies can be filled by gentleagitation of the particles across the article with periods of rest and Vbecomes essentially zero. However, the number of excess particles TW+EXis near infinity. If sufficient cleaning force is applied, all theexcess particles can be removed and TW+EX becomes zero. To be successfulthe cleaning force must be enough to remove all the excess particlesfrom the non-tacky areas yet the cleaning force must be less than theadhesive force between the particles and the tacky areas. We find thatin many cases of freshly populated tacky areas that immediate attemptsto remove the excess particles results in removing many particles fromthe tacky areas. The initial adhesion (Adh0) of the particle to thetacky areas can be very low, such that the forces applied to clean offexcess particles TW+EX exceeds the adhesive force of the particle to thetacky area, and V becomes large. This is particularly true with roughparticles that are not wet well by the tacky areas (adhesion increasesas the wetting area of the particle by the tacky area increases).

Holding the array of tacky and non-tacky areas with particles adheredthereon for a period of time and at a temperature of greater than orequal to 30° C. allows the particles to adhere and center better to thetacky areas. During this time period the surface area of the particlewet by the tacky area increases, the particle is drawn deeper into thetacky area and the particle moves toward the center of the tacky area.This process stops when the particle penetrates through the tacky areaand comes to rest in contact with the bottom of the tacky area or thecircumferential rim of the tacky area. This process is quite slow at ornear ambient temperature (e.g., 20° C.) and may take an hour or more toreach equilibrium. The time to reach equilibrium depends on severalfactors including the thickness of the tacky area, the width of thetacky area, the viscosity of the tacky material, the surface energies ofthe tacky material and particles which determines wetting rates andcharacteristics. Heating the array of tacky and non-tacky areas coveredwith particles adhered thereon greatly speeds up the wetting process andadhesion build-up of the particles to the tacky areas during the holdperiod. There is an advantage in quickly providing robust adhesion ofparticles to the tacky areas for it allows for cleaning off the excessparticles shortly after they were applied without the loss of particlesattached to the tacky areas making the overall process much moreconvenient and efficient. In some cases it may be advantageous to heatthe particles also, or to just heat the particles and not the tackyareas when the particles have sufficient thermal inertia to retain theirheat for a brief period of time until they engage a tacky area.

Suitable hold times for the methods of this invention vary withtemperature in the heating step. Illustratively, the period of time forthe hold time can broadly range from 5 seconds to 45 minutes. When thetemperature in step (d) is at least 30° C., the period of time in step(d) ranges from 10 seconds to 10 minutes. When the temperature in step(d) is at least 35° C., the period of time in step (d) ranges from 10seconds to 4 minutes. When the temperature in step (d) is at least 40°C., the period of time in step (d) ranges from 5 seconds to 60 seconds.In special embodiments where the temperature is less than 30° C., thehold time can range from 2 minutes to 1 hour.

Another surprising benefit of certain process improvements is centeringof the particles in the tacky areas during the hold period with orwithout heating. With the correct match of tacky area thickness, widthand particle geometry the wetting process that occurs during the holdperiod draws the particle to the exact center of the tacky area. It isbelieved that surface tension forces between the viscous tacky liquidand the particle surface play a dominant role in this centering process.The wetting process and centering action has been observed to occurequally well whether gravity is aiding or opposing the joining of theparticle and tacky area. That is, the process has been demonstrated withthe particle and tacky area on the topside or bottom-side of thesubstrate. This self-centering effect can be critical for aligningparticles with receptor pads in a transfer process, especially when thespacing between particles and between pads is small relative to theparticle size.

Complete centering depends on the tacky area diameters being less thanor equal to a calculated wetting or contact diameter (x) of a sphere fora particular sphere diameter (2r) and tacky area thickness(z). It isonly under these conditions that the sphere rests on the completeperimeter of the tacky area and, by definition, is completely centered.The relationships of r, x and z are illustrated in FIG. 8 and describedin the equation:r ²=(0.5×)²+(r−Z)²where:

-   sphere radius=r-   wetting diameter=x-   adhesive thickness=z-   approximate contact area=3.1416(1/2x)²

If the tacky area diameter is too much smaller than the calculatedwetting diameter, the wetted area becomes too small to achieve goodadhesive forces. For conditions where exact centering is not required,the tacky area diameter may be larger than the calculated wettingdiameter and still be substantially centered as shown in FIG. 8.Accordingly, the tacky area diameter may be 70%–170% of the calculatedwetting diameter and still work well for purposes of this invention.

A pattern of tacky dots on 50 micron thick Kapton® E film was populatedwith 125 micron solder spheres and immediately turned upside down andused to cover a hole in a sheet aluminum spacer on a microscope's hotstage. Using a combination of reflected and transmitted light the tackydots and attached solder spheres were viewed immediately through theKapton® E. The perimeter of the tacky dots and the contact area of thesolder spheres were in sharp focus while the solder sphere appears as adark shadow.

FIG. 7A illustrates the web 8 of FIG. 1 in the condition where aspherical particle 20 first engages the corner 26 of a tacky dot 28 on asubstrate 12. In FIG. 7B, which is a view looking in the direction ofarrows 7B—7B of FIG. 7A, the cross-hatched circle 30 represents thecontact area on the surface of the particle 20 that is wetted by theviscous tacky polymer of the tacky dot 28. The solid line circle 32represents the perimeter of the viscous tacky dot 28. The dashed linecircle 34 represents the particle diameter which appears as the darkshadow when actually viewing the particle through the translucent webfrom the bottom-side.

The following observations were made. The contact area 30 of the soldersphere 20 most often starts at the perimeter of the tacky dot 28 and wasinitially small relative to the area of the tacky dot. With time thecontact area of the solder sphere 20 was seen to increase as it is wetmore and more by the tacky dot 28.

FIG. 7E illustrates the condition of FIG. 7A after a substantial holdtime has taken place and wherein the relationship of the particlediameter, tacky dot diameter, and tacky dot thickness result in theparticle contacting the entire perimeter 32 of the tacky dot surfacebefore it bottoms out on the substrate 12. FIG. 7F illustrates view7F—7F of FIG. 7E.

In case of FIGS. 7E and 7F the contact area 30 grew until its perimetermatched the perimeter 32 of the tacky dot 28 in which case the soldersphere 20 had rimmed out on the surrounding non-tacky surface and wascompletely centered over the tacky dot.

FIG. 7C illustrates the condition of FIG. 7A after a substantial holdtime has taken place and wherein the relationship of the particlediameter, tacky dot diameter, and tacky dot thickness result in theparticle bottoming out on the substrate 12 before the particle contactsthe entire perimeter 32 of the tacky dot surface. FIG. 7D illustratesview 7D—7D of FIG. 7C.

In the case of FIGS. 7C and 7D the solder sphere 20 contact areaincreased until the sphere had sunk through the tacky dot 28 and restedagainst the substrate 12 of Kapton® film, in which case it had bottomedout and was partially and substantially centered on the tacky dot.

The contact area was observed until no further change was observed withtime in which case equilibrium had been reached. Time to equilibrium isa measure of the embedding rate of the solder spheres in the tacky dots.Increasing the tacky dot temperature substantially decreases the timefor embedding and centering of solder spheres in tacky dots. A 4 micronthickness is sufficient to center a 127 micron solder sphere in a 55micron tacky dot as in FIGS. 7E and 7F whereas a 3 micron thicknessresults in a bottomed out situation as in FIGS. 7C and 7D before thereis complete centering. Thus in the latter case there is only partialcentering of the sphere with respect to the tacky dot (tacky area).

Calculations show that for a 4 micron thick adhesive area and a 127micron (5 mil) sphere the tacky dot must be 44.4 microns in diameter orless for complete centering. For a 3 micron thick adhesive the tacky dotmust be 38 microns or less for complete centering. Thus with the 55micron tacky dot and 4 micron coating the 127 micron particle can be 5.3microns off center. For the 3 micron adhesive the particle can be 8.5microns off center.

Summarized below are some comparisons of observed wetting diameter atequilibrium versus calculated wetting diameter for several differentsphere diameters and coating thicknesses.

sphere coating calcd. wetting observed wetting diameter thicknessdiameter diameter (micron) (micron) (micron) (micron) 127 3.0 3837.5–42.9 127 4.0 44.4 41.6–50.0 127 6.0 53.9 127 10.0 68.4 300 4.0 68.8300 8.0 96.7 300 24.0 162.8 NOTE: calcd = calculated

The centering process continues until equilibrium is reached or theadhesive is inactivated or the particle is removed. Depending on theneed for centering in the final use of the array of tacky areaspopulated with particles, it could be advantageous to speed up thecentering and bring it nearer completion by the end of the populationprocess. Heating the surface having an array of tacky and non-tackyareas with particles adhered thereon is the best method for bothspeeding up the centering process and building adhesion between theparticles and the adhesive areas.

Significant increases in the adhesion of particles to the tacky areasoccur with a hold time of 30 to 60 inutes and some improvement isevident in 1 to 2 minutes at 21° C. For hold times of one minute or lessthe overall efficiency of the population process shows significantimprovement when the temperature is 30° C. or higher. Preferably, thepopulation of the array of tacky and non-tacky areas is conducted at atemperature that is greater than or equal to 35° C. and less than thedecomposition temperature of the tacky areas and less than the stickingtemperature of the non-tacky areas. For photopolymers described in thisinvention the decomposition temperature of the tacky areas is greaterthan 100° C. and the sticking temperature of the non-tacky areas isdependent on the degree of photocuring and on the hold time. Althoughthe non-tacky areas soften above 40° C. for a preferred composition fora light photocuring and above 60° C. for a strong photocuring,population can still be very efficient at 50° C. as long as the holdtime is short (6 seconds). Preferably, the population of the array oftacky and non-tacky areas is conducted at a temperature that is equal toor greater than 35° C. and which is less than or equal to 80° C. Morepreferably, the population of the array of tacky and non-tacky areas isconducted at a temperature that is equal to or greater than 35° C. andwhich is less than or equal to 65° C. Most preferably, the population ofthe array of tacky and non-tacky areas is conducted at a temperaturethat is equal to or greater than 35° C. and which is less than or equalto 50° C.

The particles of this invention must be free flowing particles asdefined supra, but, other than this requirement, can have any otherproperties as desired.

For many or most applications of this invention, it is desired topopulate each tacky area of an array of tacky and non-tacky areas withone and only one particle. In order to populate each tacky area with oneand only one particle, it is critical that the particle size besignificantly larger than the size of the tacky area to be populated. Ingeneral, for cases involving population of tacky areas with variousshapes, including irregular shapes, with particles of various shapes,including irregular shapes, a given tacky area should be no larger thanabout 30% of that of the particle. This value of 30% specificallyapplies for population of circular tacky areas with non-sphericalparticles. For spherical particles on circular tacky areas, it issuitable according to the invention to achieve a population of 1particle for each tacky area when each tacky area is a circle having adiameter d1 and each of the particles is a sphere having a diameter d2,wherein d1/d2 is in the range from 0.1 to 1.0. Preferably, d1/d2 is inthe range from 0.15 to 0.9. Most preferably, d1/d2 is in the range from0.3 to 0.6.

Improved Methods

The invention in one embodiment is an improved method for transferringparticles from an electrode plate to tacky areas present on a substrate.In this method, a substrate having both tacky and non-tacky areas (asdescribed supra) is placed between first and second electrode plates,with the substrate and electrode plates being arranged substantiallyhorizontally and stacked with one above or one below the other.

The particles being transferred in the method of this invention can beeither electrically conductive particles or non-conductive particles.Conductive particles are preferred for transfer in this invention.

The meaning of substantially horizontally in this invention is that eachof the indicated object(s) (e.g., substrate and electrode plates) has amain plane associated with it and that in the method of this inventionthe object is oriented such that its main plane is perpendicular to thegravitational field of Earth within 10 degrees, that is, perpendicularwithin 10 degrees to a plumb line at a given location on the Earth'ssurface.

The meaning of the terms “below” and “above” in this invention are usedin the conventional sense and are used in reference to there being astacking arrangement of the electrodes and the substrate. Specificallythis term “below” is in reference of one object being placed or locatedcloser to Earth's surface than is another object to which the first isbeing referenced. In this invention, the first electrode plate islocated closest to Earth's surface. The substrate is located furtherfrom Earth's surface, such that it is above the first electrode plate.The second electrode plate is located further from Earth's surface thanis the substrate, such that the second electrode plate is above thesubstrate.

In preferred embodiments, the first and second electrode plates in thisinvention have planar or substantially planar surfaces (for the activeelectrode area that is either at a potential or grounded). Similarly,the array of tacky areas in the substrate in preferred embodiments isplanar or substantially planar. Shapes of the electrode plates and thearray of tacky areas of the substrates otherwise are not limited—theycan, for example, be rectangular, square, circular, or ellipsoidal.

In this invention, one of the electrode plates is spaced a distance fromthe tacky and non-tacky areas of the substrate and is spaced from theother electrode plate. There is no limit to the distance for the spacingbetween the tacky and non-tacky areas and the electrode except that itmust be greater than the diameter of the largest particle(s). Thespacing between electrodes is limited by the available voltagedifference, or potential, between electrodes, and the particle weight.

The direct current potential that is applied in this invention betweenthe first and second electrode plates is at least 500 Volts, preferablyis at least 1000 Volts, and still more preferably is greater than 2000Volts. The upper limit to the direct current potential is the value thatcauses arching between electrodes at their selected spacing. The directcurrent potential is applied for a time T₁ which can be for any timeinterval longer than 0.1 millisecond. In most instances, the time T₁will be in the range from about 0.1 second to about 100 seconds,preferably in the range of 1 second to 100 seconds, more preferably inthe range of 1 second to 10 seconds, and most preferably in the range of2 seconds to 5 seconds.

In one embodiment of this invention where the particles are placed incontact with the first electrode, application of the direct currentpotential of sufficient magnitude will result in generation of anelectric field, which causes the particles to become charged with thesame charge (positive or negative) as the first electrode plate to whichthe particles are initially in contact. When the electric field is atleast a certain minimal value, the particles move upwards against theforce of gravity when the upward electrical force on the particles ishigher in magnitude than the gravitational force. The electrical forcetends to move the particles upward since the particles in contact withthe first electrode plate have the same charge as the first electrodeplate and are repelled by it. Also the particles are attracted by thesecond electrode plate when it has either the opposite charge or groundpotential. The net result is that the particles can be made to moveupwards against gravity by adjusting the direct current potential suchthat an electrical field is produced that acts on the particles with ahigher upward force than is the force of gravity acting downward(towards Earth's surface). The particles are attracted towards thesecond plate electrode, but do not contact the second plate electrodesince the particles first contact the substrate having an array of tackyand non-tacky areas that is placed between the two plate electrodes.When the particles contact the substrate having an array of tacky andnon-tacky areas, some will contact tacky areas and some will contactnon-tacky areas. Essentially all particles remain in contact with thesubstrate as long as the direct current potential is applied at the samelevel that caused the particles to move upwards. At least a portion ofthe particles will become adhered to the tacky areas of the substrateand these particles will remain adhered to the tacky areas even in theevent the potential difference between the two electrodes is changed tozero, since the substrate in this invention is chosen to have an arrayof tacky areas having sufficient tack to cause particles that contact itto remain adhered even against the force of gravity tending to cause theparticles to become unadhered (detached).

In some embodiments of the method of this invention, the method furthercomprises a step of changing the direct current potential to reverse thepolarity on the first electrode plate while the particles are adhered tothe tacky and non-tacky areas, which causes at least some of theparticles to leave the non-tacky areas of the substrate, be propelledagainst the first electrode plate, again be charged and propelled fromthe first electrode plate to the substrate. The net result is that atleast some of the particles that had been adhered to non-tacky areasprior to the polarity reversal become adhered to tacky areas at the endof the polarity reversal. When the polarity of the first electrode plateis reversed, the particles adhered to the substrate are now attracted tothe first plate electrode. Predominantly those particles adhered tonon-tacky areas will move away from the substrate to the first plateelectrode. As soon as they contact the first plate electrode, theparticles will become charged with the same charge (positive ornegative) as the first plate electrode and consequently will now berepelled from the first plate electrode. The particles now will againmove to the substrate since they are attracted at this point to thesecond plate electrode. Statistically, it is very unlikely that a givenparticle will contact the same exact spot on the substrate during thesecond contact that it did in the first contact with the substrate.There are various reasons for this, including air currents acting todisplace the particles differently in successive contacts, particlesbouncing against each other causing displacements from originaltrajectories in successive contacts, etc. Statistically, it is likelythat the particles will contact different areas of the substrate insuccessive contacts with the substrate, which provides for enhancedprobability of the particles contacting additional tacky areas andbecoming adhered with each successive contact with the substrate.

Changing the direct current potential to reverse the polarity on thefirst electrode plate is done for a time T₂ which can be for any timeinterval longer than 0.1 millisecond. In most instances, the time T₂will be in the range from about 0.1 second to about 100 seconds,preferably in the range of 1 second to 100 seconds, more preferably inthe range of 1 second to 10 seconds, and most preferably in the range of2 seconds to 5 seconds.

In some embodiments of this invention, the method further comprisesrepeating the polarity change for a number N of cycles of reversing thepolarity of the first electrode plate, whereby at least some of theparticles are repeatedly propelled against and become adhered to thesubstrate. As explained supra, each successive cycle increases theprobability of a given particle for contacting a tacky area that isunpopulated and for populating this tacky area. The net result is thateach successive cycle increases the net population efficiency in theoverall population of the tacky areas of the substrate. There is nolimit to the number N of cycles of reversing the polarity of the firstelectrode plate. In most cases, the number of cycles N is in the rangefrom 2 to 1000, preferably in the range from 10 to 100, and morepreferably in the range from 20 to 50. In some cases, N is 1, i.e., asingle cycle, with only one polarity reversal of the first electrodeplate or, in other cases, N is ½, i.e., a half cycle with a directcurrent potential being applied to the first electrode plate once withno polarity reversal(s).

In this invention, after applying a direct current potential to thefirst plate electrode and/or a number N of cycles of reversing thepolarity of the first plate electrode has been carried out, it is highlydesirable to have an efficient method for removing particles fromnon-tacky areas. One effective method for removing particles comprisesapplying ionized air to the substrate to at least partially neutralizeelectrostatic charges. The net result is that many or all particlesdisengage readily and separate from non-tacky areas once theelectrostatic charges are reduced or eliminated.

Another effective method for removing particles from the non-tacky areasof the substrate after N cycles of polarity reversals (where N is ½, 1,or a number greater than 1) is a method which further comprisesinserting a dielectric surface between the first electrode plate and thesubstrate, and spaced from the substrate, while the particles are on thesubstrate; and changing the direct current potential to reverse thepolarity on the first electrode plate, causing the particles to leavethe non-tacky areas of the substrate and be propelled against thedielectric surface. With the dielectric surface present, the particlesare prevented from again contacting the first plate electrode andchanging polarity. Once the potential difference between the twoelectrodes is brought to zero, the particles in contact with thedielectric surface readily roll off and/or can be removed from thedielectric surface.

Another way to remove particles from the non-tacky areas, which may beused separately or in addition to other methods just discussed, is tomechanically tap the substrate. The substrate with the populated tackyareas would be removed from between the electrodes and held so there issome tension in the substrate. An operator's finger, a bar, or a rod canbe tapped against the back side of the substrate opposite the tacky andnon-tacky areas to abruptly deflect the tensioned substrate and allow itto bounce back. Several such taps may be applied to facilitate removalof particles from the non-tacky areas.

FIG. 9 shows a population device 300 that can be used to process adiscrete web or substrate 302 having a surface 303 covered with arraysof tacky and non-tacky areas. A first electrode plate 326 is positionedbelow the web 302 and a second electrode plate 304 is positioned abovethe web 302. The web and plates are horizontal so particles can beplaced on the electrode and held in place by gravity. The secondelectrode 304 may include a heater 330. The web is attached to anon-conductive, annular, support ring 306 which is attached to the lowersurface 305 of second electrode 304 by taping or clamping so the web iscontacting the surface 305 which is heated by heater 330 thereby heatingthe tacky areas. Alternatively, a separate web/substrate support can beprovided (not shown) to position the web/substrate between theelectrodes and spaced from the first electrode 326. Non-conductingmaterial 307 is used to cover any exposed edges and the back side ofelectrode 304. Positioned beneath the web is first electrode 326 whichis adapted to hold a plurality of particles on a top surface 328 byplacement of a non-conductive, thin, annular ring 327 on surface 328 tokeep the particles away from the edge of the substrate. Second electrode304 is electrically connected to a DC power supply 332 by lead 301. Thepower supply 332 provides a source of DC potential and has a connectionto ground 321 and includes a switch 342 for changing the polarity of theDC power connected to the lead 301. First electrode 326 is electricallyconnected to ground 321 by lead 344. The operation of the inventionrelies on establishing an electric field between the electrodes and doesnot require a closed electrical circuit to establish current flow.Either electrode could be connected to ground; the lower electrode waschosen for safety reasons since it is more accessible to accidentalcontact than the insulated upper electrode. The first electrode 326 mayhave a heater 331 that acts to heat the particles on surface 328. Abovethe web is a vibratory tray 308 attached to a moveable frame 310 thatmoves in the direction of arrows 312 and 314 being propelled manually orby an actuator 316. The actuator may be controlled by controller 318 asis the vibratory tray 308. The tray 308 extends across the width of theweb 302 and has an outlet 309 on one side. The tray is filled withparticles, such as particle 20, to be placed on the tacky areas on theweb 302. The tray may also have a heater 311 for heating the particlesas they rest on the tray bottom. The moving and vibrating tray acts as aparticle dispenser to deliver particles 20 over the entire surface 328of the first electrode 326. The second electrode 304 is connected to anactuator 334 which is mounted to a support 320 that is attached to amachine frame (not shown). The actuator 334 is controlled by controller318 to move second electrode 304 toward and away from first electrode326. The actuator 334 acts as a plate moving device that may be attachedto the upper second electrode plate 304, as shown, or may alternativelybe attached to the lower first electrode plate 326. It functions tospace the first and second electrode plates away from each other tofacilitate delivering particles over the first plate and for attachingand removing the web/substrate 302, and for spacing the plates towardeach other to facilitate particle propulsion.

An enclosure 348 surrounds major portions of the population device asshown to contain any straying excess non-mounted particles forcollection and reuse. A container 350 is at the bottom of the enclosureto capture the excess particles.

In operation, a sheet of the web 302 is mounted on the second electrode304 with the image of tacky areas facing away from the electrode 304. Ifa cover sheet is used to protect the tacky areas of the web it would beremoved at this time and the imaged web 302 would be treated withionized air to neutralize the web. Particles such as solder sphereswould be placed in the vibratory tray 308 in a quantity greatly inexcess of what is required to populate the tacky areas. The heater 311in the tray would be continually energized to heat the particles as theyrest in the tray. Heating the particles by heaters 311 and 331illustrate a way of providing the desired heat for the process tofacilitate attachment and rapid centering of the particles on the tackyareas. Heating of the particles may be used as the sole heat source forthe process, or in conjunction with substrate (web) heating by theheater 330 in electrode 304. Alternatively, the heated electrode 304 maybe the sole source of heat in the process. Other heating means, such asradiation or convection means, may alternatively or additionally beused. What is important in all cases of heating is that the tacky areasare heated to a temperature of at least about 30° C. by whatever heatingmeans is used. The vibratory tray would be briefly cycled to distributethe spheres uniformly across the tray at the outlet. Actuator 316 wouldbe in a position to place the outlet 309 of the vibratory tray at theleft end of surface 328 of first electrode 326 as shown. The controller318 would signal the vibrator to turn on and begin dispensing particlesthat would fall from outlet 309 to the surface 328 of the firstelectrode 326. Controller 318 would signal actuator 316 to move frame310 to advance the vibratory tray from left to right so the outlet 309dispensing the spheres travels across the electrode. When the outlet 309reaches the right end of the first electrode, the vibratory tray wouldbe turned off and the actuator 316 would be reversed under the controlof controller 318 to return the tray to the left of the assembly.

After the particles are dispensed and the tray is retracted, thecontroller 318 would signal the actuator 334 to lower second electrode304 into close proximity to first electrode 326, until there is apredetermined gap between the upper electrode surface 305 and thesurface 328 of the lower electrode. The gap affects the voltage levelrequired to establish the desired electric field between the electrodes.The gap between the surface of the substrate attached to the topelectrode and the bottom electrode must still be large enough at thiselectrode gap that some velocity is imparted to the particles to createa useful lateral scattering of particles to aid in populatingefficiency. A larger gap to the substrate will result in a higher speedattained by the particles and more collisions between particles; this isbelieved to produce lateral particle motion that increases theprobability of particles encountering tacky areas. A larger gap wouldrequire a higher electrode voltage, however.

The controller 318 would now control the switch 342 on DC power supply332 to connect the negative pole (or positive pole) of the DC powersupply to lead 301. After a predetermined time, the controller wouldcontrol the switch to connect the positive pole to lead 301. After thepredetermined time, the controller would cause the switch to reverse thepolarity to the top electrode again. For solder sphere particles, apreferred predetermined time is 1–10 seconds and a more preferred timeis 2–5 seconds. When the initial polarity is established between the topand bottom electrode, the particles are propelled toward the topelectrode and attach to the substrate. For each subsequent polaritystate change (half-cycle), the particles leave the substrate on the topelectrode, contact the exposed surface of the bottom electrode and aredischarged and recharged, and they are re-attracted to the substrate onthe top electrode. After a predetermined number of polarity statechanges (which may end with a half-cycle or full cycle), the controller318 would control the switch 342 to disconnect the positive and negativepoles of power supply 332 from the lead 301 to the second electrode 304,and connect lead 301 to ground 321. The particles 20 would ordinarilyremain on the substrate 302 on the second electrode 304.

After the electrostatic cycling is complete, the web is held at rest fora predetermined time adjacent heated electrode 304. This heats the tackyareas so they will wet the surface of the particles 20 quickly whichplays a role in increasing the attachment force and the area contactwith the particle. If additional centering action is desired for aparticular set of conditions, it may be desired to continue withadditional holding time at an elevated temperature before removing thepopulated web.

The top electrode is raised by actuator 334 and the populated substrate304 would be removed from the top electrode and a new unpopulatedsubstrate could be mounted on the top electrode to repeat the populatingprocess. The populated substrate that was removed would be treated withan ionized air stream to neutralize the remaining charge on theparticles and substrate. The substrate would be gently tapped on theside opposite the particles to dislodge any particles remaining on thenon-tacky areas to depopulate them. This would complete thepopulating/depopulating process for a discrete substrate.

An alternate method to facilitate depopulating the non-tacky areas ofthe substrate 302 is to provide a non-conductive sheet 345 and anactuator 346 to insert the sheet 345 between the electrodes in the gapbetween the surface 303 of web 302 and first electrode 326 beforedisconnecting the poles from lead 301. After inserting the sheet 345,the poles would be changed for the last time which would repel theparticles from the non-tacky areas of the substrate and toward the firstelectrode 326. Since the particles would encounter the sheet 345 beforereaching the conductive surface 328 of electrode 326, the charge on theparticles would not get transferred by direct contact with the electrodeand the particles would remain on the sheet 345. The lead 301 would bedisconnected from the last pole and would be connected to ground. Theupper electrode would be raised by actuator 334 and the substrate 302populated primarily in the tacky areas would be removed. This techniquefor depopulating the non-tacky areas has been found to remove asignificant percentage of particles from the non-tacky areas, but forcomplete removal, additional depopulating may still be required.

Another variation of the invention is possible if the tacky areas aredistributed within a connected field of electrically conductivenon-tacky areas that can be used as an electrode. In this case, theapparatus of FIG. 9 would remain essentially the same as discussedexcept lead 301 would be connected to the field of non-tacky areas(non-tacky electrode) and what was formerly electrode 304 would actsimply as a support plate for the web 302. The operation would bechanged substantially in that a pole of power supply 332 would only haveto be connected by switch 342 to the non-tacky electrode once to start acontinuous oscillation of particles between the first electrode 326 andthe non-tacky electrode. As the charged particles contact the exposedelectrode surfaces of either the first electrode 326 or the non-tackyelectrode, each particle is discharged, oppositely charged, and repelledtoward the opposite electrode. The repeated propulsion toward thesubstrate with tacky areas distributed throughout the field of connectednon-tacky areas results in many opportunities for a particle to land ona tacky area and remain stuck there. After a predetermined time ornumber of oscillations, the lead 301 to the non-tacky areas isdisconnected from the DC pole and connected to ground 321. The substrateis then processed as first discussed referring to FIG. 9. In this casejust discussed, the tacky areas could also be electrically conductiveand be electrically connected to the non-tacky areas, or not, and theprocess just discussed would operate the same.

Several variations in the device are possible and still practice thepopulation process of the invention for a discrete web. Manual actuationand control can be practiced thereby eliminating controller 318. Ifheating of the particles is not required, heating means 311 (FIGS. 9 and10) and 331 (FIG. 9) may be omitted; if heating of the tacky areas isnot required, heater 330 (FIG. 9) and 331 (FIG. 10) may be omitted.Alternatively or in addition to heating with heaters 311, 331, and 330is to heat the air in enclosure 348 so all elements of the device are atan elevated temperature that would tend to heat the tacky areas andparticles. These modifications can still produce results that are animprovement over the prior art for populating particles on tacky areas.

FIG. 10 shows a population device 300 a that can be used to process acontinuous elongated web 352 having a surface 354 having repetitivearrays of tacky and non-tacky areas. In this case, the web 352 would bepresented to the device combined with a continuous elongated cover 356to form a protected composite web 358. The web 358 could be providedfrom a discrete roll 360 or could be provided from a previous webtreatment process as indicated by dashed lines 362, such as an imagingprocess. The device 300 a comprises a first web support roller 364 and asecond web support roller 366 that support web 352 horizontally over aheated first electrode 368 positioned beneath the web. The horizontalorientation allows particles to be held in place on the web by gravity.The first electrode 368 can be raised and lowered (shown lowered) soupper surface 328 can be in or out of contact with the bottom-side 400of the web 352. The plate has a heating means 331 that acts to heat theweb 352 and the tacky areas thereon. The first electrode 368 is attachedto actuators, such as actuators 336 and 338 that are attached tomounting plate 340 that is part of a machine frame. The actuators wouldbe in communication with control 318 for coordination with other machineelements. In the up position, the actuators place the upper surface 328of the heated first electrode in contact with the bottom-side 400 of theweb. When the actuators are in the down position, the surface 328 ofelectrode 368 is spaced away from the bottom-side of web 352. Keepingthe upper surface 328 out of contact with web surface 400 during webmovement is believed to minimize static buildup. Non-contact also isbelieved to facilitate the neutralization of charges on the web byionization devices to aid in depopulation of the non-tacky areas. Thefirst electrode is electrically connected by lead 344 to ground 321.

Above the web is a second electrode 369 that is at least as big as thearea covered by the tacky arrays on the web (substrate). Non-conductivematerial 371 is used to cover any exposed edges and the back side ofsecond electrode 369. Lead 301 electrically connects the secondelectrode 369 to a switch 342 on a DC power supply 332. The secondelectrode surface 373 is spaced a predetermined distance from thesurface 354 of web 352 which is under tension. When the first electrodeis contacting web 352, it establishes a predetermined gap betweensurface 328 of electrode 368 and the surface 373 of electrode 369. Thegap between electrodes affects the voltage level required to establishthe desired electric field between the electrodes. The space between thesurface 354 of the substrate and surface 373 of the second electrode 369must still be large enough at the electrode gap that some velocity isimparted to the particles to create a useful lateral scattering— ofparticles to aid in populating efficiency as discussed referring to FIG.9.

The incoming composite web 358 is additionally guided by roller 370 andis tensioned by a braking device 372 acting on roll 360. The cover 356is additionally guided by roller 374 and is collected in a discrete roll376 tensioned by a winding device 378 acting on roll 376. Positionedabove the web 352 is an ionization device 322 to neutralize staticcharges on the incoming web 352, especially that generated duringstripping of cover sheet 356. Device 322 is directed at the positionwhere cover 356 is peeled off web 352. Also above the web 352 is avibratory tray 308 having an outlet 309 and heating means 311 as in FIG.8 for dispensing particles 20. The vibratory tray is fixed to a machineframe at position 380. Web 352 is further guided by rollers 382, 384,386, and 388 before passing between driven roller 390 and nip roller392. The web 352 passes under nip roller 392 with the tacky area surface354 facing roller 392 which is relieved in the central portion to avoidcontact with any of the populated tacky areas. Cutting means 394 andholding table 396 are adjacent driven roller 390 and in the path of web352.

Between rollers 382 and 384, the web is transported horizontally andpasses between ionizing air knife 324 directed at the web surface 354,and ionizing air knife 324 a directed at the opposite web surface 354 b.The air knife is a known device that uses a row of ac corona dischargeneedles to ionize the surrounding air in a band. A sheet-like stream offlowing air is directed past the needles to forcefully distribute theionized air over the web surface. The corona discharge function can alsobe used effectively if separated from the air function when air flow isnot desired. The devices 324 and 324 a extend across the width of theweb 352. Tapping device 404 is positioned to be able to contact side 354b of web 352 opposite the tacky and non-tacky arrays. The tapping devicehas a moveable member 406 that extends out to contact the web whendesired. The tapping acts to mechanically dislodge particles on thenon-tacky areas of the web. A tapping impact frequency of 1–2 taps persecond and a tapping amplitude of 0.03–0.3 inches web movement has beenfound to be effective for depopulating the non-tacky areas. Beneath thedevice 300 a is a container 398 for collecting excess particles 20.Controller 318 is used to control the various elements of the device 300a.

In operation of the device 300 a, an elongated composite web 358, havingmultiple repeating tacky and non-tacky arrays imaged thereon, would beprovided from roll 360 and would be threaded over roller 370 and supportroller 364 that acts as a first transport roller for transporting theweb between the electrodes. Cover 356 would be peeled off of thecomposite web at roller 364 leaving web 352 to proceed to support roller366 that acts as a second transport roller for transporting the webbetween the electrodes. Cover 356 would proceed over roller 374 to roll376 where it will be wound, driven by winding device 378. Web 352 wouldbe threaded around rollers 382, 384, 386, and 388 and through the nipformed by driven roller 390 and nip roller 392. Driven roller 390 may bepropelled by a drive such as a servo motor, stepping motor, or the likeunder the control of controller 318 to achieve precise movement of web352. Control of braking device 372 by controller 318 will providetension control for web 352 and composite web 358. Control of windingdevice 378 by control 318 will provide tension control for web 356.

The web is stopped to position a complete tacky area array over heatedfirst electrode 368 and under second electrode 369 and another adjacentarray under ionization device 322. During advance of the web 352, theelectrode 368 is retracted to avoid rubbing contact with web 352 whichwould generate electrostatic charges that would be difficult toneutralize. It is significant that the web 352 is at one point notcontacting any surfaces between support rollers 364 and 366 to therebyprovide good conditions for electrostatic charge neutralization. Duringadvance of the web 352, vibratory tray 308 is energized to dispenseparticles 20 through outlet 309 to fall onto the static neutralized web352. As the particles are cascading onto web 352, one repeat of themultiple tacky arrays on web 352 passes by the outlet 309 so one entirearray is covered by this relative motion between web 352 and outlet 309.When the covered array stops over electrode 368, the vibratory tray isdeenergized and the flow of particles from outlet 309 stops. It may bedesirable to position the outlet 309 so that when the web 352 stops, theoutlet is over a gap between multiple arrays. The rollers 390 and 392,cutting means 394, and holding table 396 are preferably positioned sothat when the array stops over plate 368, a populated array also ispositioned with the gap between arrays located at the cutting means 394.The cutting means can then be actuated by controller 318 to cut betweenthe arrays and thereby separate one populated array from the continuousweb 352 as desired for further handling.

When the covered array stops over electrode 368, actuators 336 and 338are signaled by controller 318 to raise electrode 368 to contact thebottom-side 400 of web 352 and establish the predetermined electrode gapbetween electrodes 368 and 369. The heated surface 328 of electrode 368quickly heats the tacky areas on the web. The controller 318 would nowcontrol the switch 342 on DC power supply 332 to connect the negativepole (or positive pole) of the DC power supply to lead 301. This resultsin particles being propelled off the web 352, contacting electrodesurface 373, and being propelled back to web 352 to thereby populatepreviously unpopulated tacky areas thereon. After a predetermined time,the controller would control the switch to connect the positive pole tolead 301 which would again cause propulsion of particles to surface 373and back to surface 352. After the predetermined time, the controllerwould cause the switch to reverse the polarity to the top electrodeagain as discussed referring to FIG. 9. After a predetermined number ofpolarity cycles, the lead 301 is disconnected from the power supplypoles and is connected to ground 321. The web is held at rest in contactwith heated surface 328 for a predetermined time during which the web isheated and the particles on the tacky arrays are firmly adhered andcentered in each tacky area. After the predetermined hold time, theelectrode 368 is retracted out of contact with web 352 and thecontroller causes driven roller 390 to advance the web a distance of onetacky array. As the just populated array passes over support roller 366,the particles on the non-tacky areas of the array progressively cascadedown off the web and are collected in container 398. The progressivecascading and the angled web path at 402 prevent a large quantity ofparticles from coming off the web all at once that might dislodge theparticles attached to the tacky areas. The flexibility of the webpermits this progressive change in path over roller 366.

As the web with previously populated arrays is moving between rollers382 and 384 the controller turns on air flow to air knives 324 and 324 apositioned between the rollers. The ac corona to the air knives mayremain on continuously. Air knife 324 acts to blow off excess particlesthat may still be temporarily adhered to the non-tacky areas as themoving web 352 passes by knife 324. During web movement, the controller318 also directs tapping device 404 to tap the web for a predeterminedtime or number of taps. Air knife 324 a similarly acts to blow off anyparticles that may have inadvertently fallen onto the back-side of theweb 352. When the web motion stops for the next cycle, the air-flows toair knives 324 and 324 a are turned off by controller 318. Tappingdevice 404 could alternatively be actuated for its predeterminedduration only during the time the web motion is stopped.

After stopping the web motion, controller 318 also activates holdingtable 396 to grasp populated web 352 with a vacuum while cutting means394 is cycled to cut the web between populated arrays. The entire cyclejust described can now repeat for the next tacky array on the continuouselongated web. Such an automated device 300 a for populating acontinuous web offers productivity advantages and labor savings notpossible before.

Referring to FIG. 10, there can be several variations to the hold timefor the populated web in the process. A first hold time may occurbeginning just after the particles have been agitated by the electrodesand the web is resting on heated electrode 368 and before the web isindexed off electrode 368 to present a new array for populating. Duringthis time no forces are applied to the excess particles to try to removethem that may result in disturbing the particles on the tacky areas. Asecond hold time may occur beginning just after the web has beenadvanced to move the just populated array off the electrode 368 and overthe roller 366. Many of the excess particles will fall off the web dueto gravity as the web is bent over roller 366, but the excess particleswill not yet have been aggressively removed by air jets or vibrations(tapping). During this hold time the particles on the tacky areas havenot been disturbed and may still be undergoing additional wetting by thetacky material to improve adhesion and centering. This second hold timeextends until the populated array is advanced past the air knife 324during one of the web advances. A third holding time may occur beginningjust after the excess particles have been aggressively removed by airknife 324 and before the array leaves the apparatus environment afterrollers 390 and 392. The first, second, and third hold times arecontrolled times when the populated web may be treated withindependently controlled heating means, or may not be heated, forpredetermined times to improve adhesion and centering of the particles,if they have not yet reached the limits of centering, before thepopulated web is handled for further use.

Variations in the device 300 a are possible and still practice thepopulation process of the invention. For instance, different heatingmeans may be employed to heat the tacky areas between support rollers364 and 366. Hot air convection heating may be employed with the airdirected at the surface 354 and/or back-side 400. Radiant heating mayalso be alternatively employed or employed in combination with otherheating means and directed at the surface 354 and/or back-side 400 ofweb 352. When these alternate variations are employed, electrode 368 maynot be heated and it may be spaced from the web.

The process just described referring to FIG. 10 where the substrate isplaced over the first electrode and covered with particles can also bepracticed with a discrete substrate instead of a continuous substrate.Referring to FIG. 9, the substrate 302 can be removed from electrode 304and placed on surface 328 of first electrode 326 with the tacky andnon-tacky areas facing electrode 304. The particles would be distributedover substrate 302 and the DC pole connected to electrode 304. Thepopulating would take place as described for the similar setup referringto FIG. 10. Depopulation would take place as already described for FIG.9 by removing the substrate from electrode 326 and going through thedepopulating steps.

The population methods of this invention will afford populated surfaceshaving an array of tacky and non-tacky areas in which almost all of thetacky areas of the surface are populated with one particle per tackyarea upon completion of execution of the method. Typically, there willbe at least 99.99% of tacky areas of the surface populated with oneparticle per tacky area.

The population methods of this invention will afford populated surfaceshaving an array of tacky and non-tacky areas in which very few particlesremain attached to non-tacky areas upon completion of execution of agiven method. Typically, there will be fewer particles than one per10,000 that remain in the non-tacky areas.

EXAMPLES Example C-1

The photosenstive layer of the unimaged tacky dot film used in theexamples that follow had the following composition:

Ingredient Amount (g) % by Weight Poly(methyl methacrylate), 6.97 12.18MW* = ~250,000 Poly(methyl methacrylate), 9.39 16.41 MW* = ~20–40,000Pentaerythritol triacrylate 14.54 25.41 Tetraethylene glycoldimethacrylate 9.02 15.77 Monoacrylate of resin from bisphenol A andepichlorohydrin, MW* = ~3,500 12.53 21.902,2′-Bis(o-chlorophenyl)-4,4′,5,5′- 4.18 7.31tetraphenyl-1,2′-biimidazole 4,4′-Bis(diethylamino)benzophenone 0.2510.44 Leuco Crystal Violet 0.275 0.48 (Aldrich Chemical Co., Milwaukee,WI) 1,4,4-Trimethyl-2,3-diazobicyclo- 0.0286 0.05(3.2.3)-non-2-ene-dioxide 4-Methoxyphenol 0.0286 0.05 TOTAL 57.2132 *MW= weight average molecular weight

The photosensitive layer was coated onto Kapton® E (50 micronsthickness, DuPont, Wilmington, Del.) and dried to give a dry coatingthickness of the photosensitive layer of 3 to 25 microns.

The unimaged tacky dot film was imaged in the examples using contactexposure through a phototool by ultraviolet light at 365 nm and exposurelevel of 5 to 20 millijoules/cm2.

Example 1

This example illustrates a basic process and apparatus used forelectrostatic populating. Two electrodes were used, each comprising afive inch square aluminum plate ⅛″ thick. One plate was used as a bottomelectrode and was left completely uncovered and was connected to aground on a high voltage power supply. The other plate was used as a topelectrode and was covered completely on one side and the edges andpartially covered on the other side with Kapton® tape, which is anon-conductive covering. The partially covered side had a centralopening about 4 inches square where the electrode was uncovered. The topelectrode was arranged to be connected to either the positive ornegative lead of a high voltage power supply which was as follows:

-   -   Hipotronics Model 8100B (Brewster, N.Y.)    -   0–100 kilovolts    -   0–5 milliamperes    -   DC, reversible, continuous duty, 2% rms ripple    -   115 VAC, 50/60 cycle input

The particles to be populated on the substrate are 5 mil diameter solderspheres composed of 63% tin and 37% lead. They were obtained from IndiumCorp. of Utica, N.Y.

Process:

A non-conductive substrate having a photopolymer adhesive having anarray of tacky and non-tacky areas covering an area of about 4 inches by4 inches was taped over the 4 inch by 4 inch exposed area of the topelectrode plate so that none of the top electrode was exposed. The tackyareas were facing outward. The process was carried out at ambientconditions without any special air filtration or humidity or temperaturecontrols.

A quantity of 5 mil diameter solder spheres (in excess of the numberrequired to populate all tacky areas with at least one sphere) wasapproximately uniformly distributed over the surface of the bottomelectrode plate in a one to two sphere thick layer. The top electrodeplate was placed at a gap of 0.050 inches from the surface of the bottomelectrode, which affects the voltage level to establish the field. Thegap between the surface of the substrate attached to the top electrodeand the bottom electrode must still be large enough at this electrodegap that some lateral velocity is imparted to the spheres to create auseful lateral scattering of spheres to aid in populating efficiency.The bottom plate was grounded and the top plate was contacted with thenegative lead at a potential of 4 kilovolts DC which was held to the topplate for a period of time. The solder spheres were observed to leavethe bottom electrode and be propelled toward the top electrode andimpact the substrate having an array of tacky and non-tacky areas. Someof the spheres were propelled immediately and some were propelled latersuch that movement of the spheres between the bottom electrode andsubstrate on the top electrode made a sound that resembled falling rain.After about 2–5 seconds, the motion of the spheres was greatly reducedand the sound stopped.

The negative lead was removed from the top electrode and the positivelead at a potential of 4 kilovolts DC was brought into contact with thetop electrode. The spheres were repelled away from the top electrode andafter contacting the bottom electrode, they were propelled once again tothe top electrode. The “raining” movement of the spheres was repeated asthe positive lead was maintained in contact for 2–5 seconds. Thepositive lead was removed at this point from the top electrode. Theabove completes and defines one cycle of polarity reversals from zero topositive to negative and back to zero. This cycle of polarity reversalsresulted in the spheres being propelled twice from the bottom toward thetop electrode. At the end of the above cycle, the spheres were observedto reside on the substrate attached to the top electrode. The topelectrode was now grounded to remove any residual voltage. Thisgrounding had little effect on the spheres which are believed to have astatic charge that resists the effects of gravity on the spheres.

After removal of the substrate it was noted that spheres were attachedto both tacky and non-tacky areas. The substrate was mechanically tappedon the side opposite the spheres and most of the balls attached tonon-tacky areas were dislodged. The tapping was accomplished by theoperator flicking his finger against the substrate held along one edgeand hanging downward. About 4–6 finger flicks were applied about 1second apart to dislodge the balls from the non-tacky areas. Thepopulation efficiency was analyzed by dividing the number of correctlypopulated tacky area sites by the total number of sites. No attempt wasmade to account for failure to populate due to dust or othercontamination of tacky dot sites, nor was any adjustment included forextra spheres in background non-tacky areas. The population efficiencywas 92%, which was considered good for the uncontrolled conditions used.

Examples 2–4

To explore the effect of number of propulsion cycles upon the populationefficiency, the following three tests were run (Examples 2–4).

Example 2

In Example 2, the lower limit of cycles possible was tested which isone-half a cycle. The setup of Example 1 was used. The negative lead wascontacted to the top electrode for 2–5 seconds until the “raining” ofspheres from the bottom toward the top electrode stopped. The negativelead was removed from the top electrode and the electrode was grounded.The substrate was removed and tapped, and the population efficiency wasthen measured as in Example 1. The population efficiency was found to be69%, which was significantly less than that in Example 1. It is believedthat only one propulsion of spheres toward the substrate does not affordenough opportunities for at least one sphere to contact each tacky dotin the array.

Example 3

The setup of Example 1 was used. The negative lead was contacted to thetop electrode for 2–5 seconds until the “raining” of spheres from thebottom toward the top electrode had stopped. The negative lead was thenremoved and the positive lead connected for 2–5 seconds as in Example 1.This sequence was repeated five times so that 5 cycles of polarityreversals occurred that propelled the spheres from the bottom electrodetoward the top electrode a total of 10 times (2 times/cycle and 5cycles). At the end of the fifth cycle, the lead was removed from thetop electrode and it was grounded as in Example 1. The substrate wasremoved and tapped and the population efficiency was measured as inExample 1. The population efficiency was 93%, which was slightly betterthan that in Example 1.

Example 4

The setup of Example 1 was used. The negative lead was contacted to thetop electrode for 2–5 seconds until the “raining” of spheres from thebottom toward the top electrode had stopped. The negative lead was thenremoved and the positive lead connected for 2–5 seconds as in Example 1.This sequence was repeated ten times so that 10 cycles of polarityreversals occurred that propelled the spheres from the bottom electrodetoward the top electrode a total of 20 times (2 times/cycle and 10cycles). At the end of the tenth cycle, the lead was removed from thetop electrode and it was grounded as in Example 1. The substrate wasremoved and tapped and the population efficiency was measured as inExample 1. The population efficiency was 95%, which was believed to besignificantly better than that in Example 1.

Example 5

Tests were run to determine the applicability of using an electrostaticparticle propulsion process for propelling electrically non-conductiveparticles, (unlike the spheres used in Examples 1–4, which were allelectrically conductive). As this test was not identical to that inExamples 1–4, the spheres were included as a point of comparison. Inthis series of tests, the substrate was omitted as was thenon-conductive covering on the surface of the top electrode facing thebottom electrode. The top electrode was 12 inches square and the bottomwas 6 inches square. The electrodes were spaced apart at two distancesdepending on the particles and voltage used. Particles were initiallyplaced on the bottom electrode which was grounded, and the top electrodewas connected to either the negative or the positive lead of the powersupply. Testing indicated the polarity of the lead did not affect theresults. Upon connecting the selected lead to the top electrode, theparticles were propelled to the top electrode. As soon as the particlescontacted the exposed top electrode, they were repelled back to thebottom electrode from which they were propelled back again to the topelectrode. This oscillation or bouncing of the particles between the topand bottom electrode continued until the particles bounced out frombetween the electrodes, became attached to one electrode, or until thelead was removed from the top electrode. The following table summarizesthe results that were obtained.

Size Density Gap Voltage (KV) to Levitate Particle Micron g/cc mm andInitiate Bouncing solder spheres 127 ~9 28.1 13.5 solder spheres 127 ~915.8 7.0 polystyrene spheres 113 1.06 15.8 4.0 glass spheres 125 2.4815.8 4.0

The solder spheres and glass spheres oscillated about 2–20 seconds untilmost had bounced out from between the electrodes aided by air currents.The polystyrene spheres were observed to eventually stop bouncingbetween electrodes after about 10–40 seconds and to cling to the twoelectrodes. Somewhere between about ⅓ to ⅔ of the particles weredistributed on each electrode. It is believed this occurred with thepolystyrene due to high charging and the inability to quickly dischargethe polystyrene. The example demonstrates the ability of electric fieldsto levitate both conductive and non-conductive particles.

Example 6

This example illustrates the effect of a conducting ground plane on themobility and electrostatic charging of eutectic solder spheres rollingacross a photocured polymeric coating with and without an electricallyconducting aluminum ground plane between the coating and a polyesterfilm support.

The coating was applied by lamination as a 4 micron thick layer of tackyphotopolymer having the composition below:

Ingredient Amount (g) % by Weight Poly(methyl methacrylate), 6.97 12.18MW* = ~250,000 Poly(methyl methacrylate), 9.39 16.41 MW* = ~20–40,000Pentaerythritol triacrylate 14.54 25.41 Tetraethylene glycoldimethacrylate 9.02 15.77 Monoacrylate of resin from bisphenol A 12.5321.90 and epichlorohydrin, MW* = ~3,5002,2′-Bis(o-chlorophenyl)-4,4′,5,5′- 4.18 7.31 tetraphenyl-biimidazole4,4′-Bis(diethylamino)benzophenone 0.251 0.44 Leuco Crystal Violet 0.2750.48 (Aldrich Chemical Co., Milwaukee, WI)1,4,4-Trimethyl-2,3-diazobicyclo- 0.0286 0.05 (3.2.3)-non-2-ene-dioxide4-Methoxyphenol 0.0286 0.05 TOTAL 57.2132 *MW = weight average molecularweightThe photosensitive composition is suitable to give a dry coatingthickness on a substrate of 3 to 25 microns.

The coating was applied as a 4 micron thick layer of tacky photopolymeronto 2 mil thick aluminized Mylar® for a first sample, and was appliedby coating as a 4 micron thick layer of tacky photopolymer onto plainMylar® film for a second sample. The coating was covered by a 0.5 milpolyester film coversheet. The unimaged tacky dot film was imaged usingcontact exposure through a phototool. The coating was photocured byultraviolet irradiation of sufficient intensity and duration to make thecoating non-tacky, for instance ultraviolet light at 365 nm and exposurelevel of 5 to 20 millijoules/cm2.

The coated samples were cut to fit the bottom and sides of a six inchlong inclined plane sample holder. The samples were folded at each sideto cover the side of the sample holder. The coversheet was removed fromthe photocured polymer coating and the surface discharged by ionizedair. The sample then was placed in the inclined holder.

Solder spheres 5 mils in diameter consisting of 37% lead, 63% tin fromIndium Corporation of America, Utica, N.Y. were conveyed by a vibratoryfeeder (Syntron Magnetic Feeder, Model F-TO, FMC Corporation, MaterialHandling Division, Homer City, Pa.), dropped onto the top of theinclined plane sample holder heated at 50° C., and rolled into a Faradaycup (Model 253/22B, FMC Corporation, Material Handling Division, HomerCity, Pa.) to measure electrostatic charge on the spheres. Drop height dbetween the lip of the feeder and the top of the sample holder wasvaried from 2.5 to 14 centimeters. Sample holder incline angle (theta)was varied from 15 to 45 degrees. Critical observations were:

1) whether all the solder spheres rolled across the inclined plane intothe Faraday cup and

2) the electrostatic charge generated on the spheres by rolling acrossthe sample.

All experiments were at 50% relative humidity.

FIG. 11 illustrates the equipment used in this Example 6. A vibratoryfeeder 440 with electrically conductive tray 445 is mounted on a labjack 450 such that the drop height d can be varied. An electricallyconductive sample holder 430 was constructed such that the sample holderincline angle theta (E)) could be varied. The sample 410 containing anarray of tacky and non-tacky areas was placed on the sample holder 430as shown in section view of the sample holder and sample. The vibratoryfeeder 440 and sample holder 430 were both grounded with grounding clips435. Solder spheres exited the end 420 of the vibratory feeder 440,dropped a distance d to contact the sample 410, and then rolled down andoff the sample 410 along a solder sphere path 470 to the Faraday Cup400, where measurement of the electrostatic charge on the spheres wasmade. The complete apparatus was contained in two carbon fiberconductive trays 460.

It is clear from the experiments that samples without the conductingground plane allowed some spheres to stick electrostatically and gavehigher electrostatic charge to the spheres. None of the spheres stuckelectrostatically while rolling down samples with the aluminum groundplane.

Sphere charge Aluminum Drop Angle (nonocoulombs Spheres elect-statGround (cm) (deg) per gram) stuck none 14 30 n2.6–4.2 none none 14 45n3.1–4.4 none yes 14 30 n2.6–3.0 none yes 14 45 n2.0–2.6 none none 10 30— many stuck none 10 45 n7.7–8.8 none none 10 45 n3.8–6.3* some yes 1030 n3.3–4.0 none yes 10 45 n3.7–4.1 none none 2.5 15 — most stuck none2.5 30 p1.8–n3.3* some none 2.5 45 n0.1–3.5* some yes 2.5 15 n0.1–0.3none yes 2.5 30 n0.4–0.6 none yes 2.5 45 n1.6–2.1 none *Sphere chargeper gram inaccurate when some electrostatically stick. Actual charge pergram is larger. n = negative charge; p = positive charge — Indicatesthat in this case most spheres stuck to the film, and did not fall intothe Faraday Cup. Consequently the reading was very low and veryinaccurate. The information gained was that the static was so great thatit caused the spheres to stick to the film, under the conditions aslisted. NOTE: In some tests not recorded above in Example 6, a groundingclip was attached to the conductive aluminum coating on the sample, butit was not observed to change the performance significantly.

1. A method for transferring particles from an electrode plate to tackyareas present on a substrate, comprising: a) placing a substrate havingboth tacky and non-tacky areas between first and second electrodeplates, the substrate and electrode plates arranged substantiallyhorizontally and stacked substantially vertically, wherein the firstelectrode plate (i) lies below the substrate, (ii) has a surface whichfaces tacky and non-tacky areas on the substrate, and (iii) is spacedfrom the substrate and the second electrode plate; b) applying particlesover the surface of the first electrode plate; c) applying a directcurrent potential between the first and second electrode plates for atime T₁, establishing a polarity on the first electrode and therebycausing the particles to be charged, and be propelled toward the secondelectrode plate and to the substrate resulting in at least a portion ofthe charged particles becoming adhered to tacky areas on the substrate;and d) changing the direct current potential to reverse the polarity onthe first electrode plate for a time T₂ after step c) to cause at leastsome of the particles to leave the non-tacky areas of the substrate, bepropelled against the first electrode plate, again be charged andpropelled from the first electrode plate to the substrate, resulting inat least some of the particles that had been adhered to non-tacky areasprior to step d) being adhered to the tacky areas at the end of step d).2. The method of claim 1, further comprising the step of: e) repeatingin sequence steps c) and d) for a number N of cycles of reversing thepolarity of the first electrode plate whereby at least some of theparticles are repeatedly propelled against and become adhered to thesubstrate.
 3. The method of claim 2, further comprising the steps of: f)eliminating the direct current potential between the electrode plates;and g) removing particles from the non-tacky areas of the substrate. 4.The method of claim 1 wherein only a single particle becomes adhered toeach tacky area.
 5. The method of claim 1 wherein the substrate havingboth tacky and non-tacky areas comprises electrically non-conductivematerial and the particles comprise electrically conductive material. 6.The method of claim 3, wherein removing the particles comprises applyingionized air to the tacky and non-tacky areas on the substrate to atleast partially neutralize electrostatic charges.
 7. The method of claim1, wherein the direct current potential between the first and secondelectrode plate is greater than 500 volts.
 8. The method of claim 1,wherein the time T₁ and the time T₂ for each of steps c) and d) areindependently in the range of 0.1 second to 100 seconds.
 9. The methodof claim 2, wherein the number N of cycles of reversing the polarity ofthe first electrode plate is in the range of from 2 to
 1000. 10. Themethod of claim 2, wherein the number N of cycles of reversing thepolarity of the first electrode plate is in the range of from 10 to 100,and wherein the time T₁ and the time T₂ for each of steps c) and d) areindependently in the range of 1 second to 100 seconds.
 11. The method ofclaim 2, wherein the number N of cycles of reversing the polarity of thefirst electrode plate is in the range of from 20 to 50, and wherein thetime T₁ and the time T₂ for each of steps c) are d) are independently inthe range of 2 seconds to 5 seconds.
 12. The method of claim 3, whereinremoving the particles from the non-tacky areas further comprises: h)prior to steps f) and g), inserting a dielectric surface between thefirst electrode plate and the substrate, and spaced from the substrate,while the particles are on the substrate; and i) changing the directcurrent potential to reverse the polarity on the first electrode plate,causing the particles to leave the non-tacky areas of the substrate andbe propelled against the dielectric surface, the particles free ofcontact with the first electrode plate.
 13. A method for mountingparticles on a substrate having both tacky and non-tacky areas thereon,comprising the steps of: a) placing a first electrode plate orientedsubstantially horizontally and spaced from a second electrode plate, thefirst electrode plate lying above the second electrode plate and havinga conductive surface facing the second electrode plate; b) placing asubstrate between the first and second electrode plates and spaced fromthe first electrode plate with the tacky and non-tacky areas of thesubstrate facing the first electrode plate; c) covering the substratewith particles to thereby populate at least some of the tacky areas withparticles; d) applying a direct current potential between the first andsecond electrode plates to establish a polarity on the first electrodeplate for a time T₁, wherein the particles in the non-tacky areas areattracted and propelled to the first electrode plate, where theparticles are charged and thence propelled from the first electrodeplate back to the substrate; and e) changing the direct currentpotential to reverse the polarity on the first electrode plate afterstep d) for a time T₂; wherein the particles are caused to move awayfrom the non-tacky areas of the substrate, be propelled against thefirst electrode plate, again be charged and repelled from the firstelectrode plate, and be propelled against the substrate.
 14. The methodof claim 13, further comprising the step of: f) repeating in sequencesteps d) and e) for a predetermined number N of polarity reversalswhereby at least some of the particles are repeatedly propelled againstthe electrode plate, are charged and repelled, and are propelled againagainst the mounting surface and become adhered to the tacky areasthereon, the population efficiency increasing with each polarityreversal.
 15. The method of claim 12, further comprising the step of: j)heating the tacky areas to a temperature of at least 30° C.
 16. Themethod of claim 14, further comprising the step of: g) heating the tackyareas to a temperature of at least 30° C.
 17. The method of claim 1,wherein the direct current potential between the first and secondelectrode plates is greater than 500 volts.